In order to function in a wireless communication network, a UE (User Equipment) must connect to a cell provided by an RBS (Radio Base Station). In order to access a cell, the UE performs a so-called cell search procedure. By performing a cell search procedure, the UE acquires time and frequency parameters needed in order to correctly demodulate data from an RBS. The parameters acquired during this procedure are in particular e.g.: a symbol start position, a carrier frequency and sampling clock synchronization.
The cell search procedure depends on whether the UE is performing an initial synchronization, i.e. a first detection of a cell and decoding of information needed in order to register to it, or is performing a new cell identification, i.e. the detection of neighboring cells, e.g. as a preparation for handover, when the UE is already connected to a serving cell.
In each cell in an LTE-type system, at least two signals are broadcasted by the eNB serving the cell in order to facilitate the cell search procedure: the so-called Primary Synchronization Signal (PSS) and the so-called Secondary Synchronization Signal (SSS).
Primary Synchronization Signal (PSS)
During an initial cell search, a UE first searches for the primary synchronization signal (PSS) which is transmitted in the last OFDM symbol of the first time slot of the first subframe (subframe 0) in a radio frame, cf. FIG. 1. The PSS enables the UE to acquire the slot boundary independently from the chosen cyclic prefix selected for this cell. Based on the downlink frame structure, e.g. Type 1, FDD (Frequency Division Duplexing), the PSS is transmitted twice per radio frame, e.g. first in subframe 0, and is then repeated in subframe 5 (in time slot 11). This enables the UE to get time synchronized on a 5 ms basis. The location of the PSS within the bandwidth is also illustrated in more detail in FIG. 2.
Secondary Synchronization Signal (SSS)
After that a UE has found the 5 ms timing by use of the PSS, it needs to obtain the radio frame timing and the cell's group identity. This information can be derived from the SSS. When regarded in the time domain, the SSS is typically transmitted in the symbol before the PSS, which is also illustrated in FIG. 1. The SSS, like the PSS, also has a 5 ms periodicity. When the SSS is transmitted in the symbol preceding the PSS, this means that the SSS is transmitted in the first and sixth subframes, denoted subframes 0 and 5. Like the PSS, the SSS is typically transmitted on 62 of the 72 reserved subcarriers around the DC subcarrier.
A PCI to be used in a cell could be derived and assigned to an RBS based on a PSS and an SSS, e.g. according to the formula: PCI=3×SSS+(Spacing×PSS); where “spacing” is a parameter which is selected in order to avoid that adjacent cells get the same frequency shift.
Various strategies may be used to assign or allocate a PCI (Physical Cell Identity) to a cell, e.g. in an LTE network. The choice of the allocation scheme has impact on processing requirements and synchronization performance at the UE. There are a number of PCI planning tools available in the market. However, the performance of these tools is unsatisfactory, and may result in PCI confusion in a system where they are used. The algorithms of these existing PCI planning tools are often based on coverage prediction.